1. Field of the Invention
The present invention relates generally to self-oscillation type converters. More particularly, the present invention relates to self-oscillation type converters for use in a constant voltage power supply circuit and the like.
2. Description of the Background Art
Conventionally, a self-oscillation type DC-DC converter is generally used as a low-price switching regulator having a relatively low output capacity.
FIG. 1 is an electric circuit diagram of a conventional self-oscillation type DC-DC converter having one transistor which is generally referred to as a ringing choke converter. Referring to FIG. 1, a transformer 1 includes a primary winding Np, a secondary winding Ns and a feedback winding Nb, and one end of the primary winding Np is connected to a first input power supply line and the other end thereof is connected to a collector of a switching transistor Q1. One end of the feedback winding Nb is connected to a base of the switching transistor Q1 through a diode D2 and a resistor R3. The other end of the feedback winding Nb is connected to an emitter of the switching transistor 01 and a second input power supply line. A start resistor R2 is connected between the base of the switching transistor Q1 and the first input power supply line. In addition, a rectifying diode D1 is connected in series with the secondary winding Ns of the transformer 1 and a smoothing capacitor C2 is connected in parallel.
Now, the operation of the self-oscillation type DC-DC converter shown in FIG. 1 will be described. First, an application of a voltage to the input power supply line causes a current flow through the start resistor R2 to the base of the switching transistor Q1, rendering the switching transistor Q1 conductive. As a result, a voltage is applied to the primary winding Np of the transformer 1 and at the same time a voltage is generated in the feedback winding Nb. The voltage generated in the feedback winding Nb becomes a positive feedback voltage which further renders the switching transistor Q1 conductive, whereby the switching transistor Q1 is rapidly turned on. At this time, a voltage of the secondary winding Ns of the transformer 1 is applied to the diode D1 in a reverse direction, and energy is stored in the transformer 1 by the current flowing through the primary winding Np. Thereafter, when the base current cannot maintain the saturation state of the switching transistor Q1 due to the increased current of the switching transistor Q1, the collector-emitter voltage of the switching transistor Q1 is increased, so that the voltage of the primary winding Np of the transformer 1 falls. As a result, a voltage of the feedback winding Nb also falls. Since this change is positively feed back to the switching transistor Q1, the switching transistor Q1 is rapidly turned off. After the switching transistor Q1 is turned off, the energy stored in the transformer 1 is supplied to the output side through the rectifying diode D1.
As described in the foregoing, a positive feedback of the feedback winding Nb of the transformer 1 to the base of the switching transistor Q1 enables a self-oscillation type converter to be structured by simple circuits.
The general ringing choke converter shown in FIG. 1 is not provided with a stable protecting function against an over-load. Assuming that the maximum base current of the switching transistor Q1 is constant, the maximum collector current depends on the hfe (direct current amplification rate) of the switching transistor Q1. Accordingly, although this hfe serves as a protecting function against the over-load, the hfe varies greatly among switching transistors, which prevents a stable protecting operation against the over-load.